The present invention relates to pressure sensitive adhesives (PSAs) and self-adhesive labels. More particularly, the invention provides acrylic PSAs and constructions exhibiting improved convertibility and reduced adhesive build-up in printers.
PSAs and self-adhesive labels are now used extensively in the home, in the office and in many commercial settings. In a typical construction, a removable release liner is coated with a PSA, which is laminated to a label face stockxe2x80x94typically an imprintable paper or plastic. Removal of the release liner allows the label to be adhered to a substrate. Such constructions are typically produced in large (e.g., 79 in. wide) rolls, which are then slit into smaller (e.g., 11 in. wide) rolls. Label sheets are made by cutting the resulting rolls into sheets, for example, 8xc2xdxe2x80x3xc3x9711xe2x80x3 sheets, A4 sheets or 5xe2x80x3xc3x978xe2x80x3 sheets. Individual labels are fabricated by die-cutting and sheeting the construction.
A common problem with sheets of PSA labels is edge tackiness at the slit ends of the sheets, caused by the tendency of the PSA to flow or xe2x80x9coozexe2x80x9d under pressure, such as the pressures encountered during the slitting process and while the constructions are wound up in rolls. Edge tackiness of label sheets tends to adversely affect the performance of computer printers and copiers, which were really designed for printing on paper. The problem is particularly acute with thermal printers, such as laser printers and copy machines. Multi-layer sheets of self-adhesive labels tend to leave an adhesive residue in the printer in various locations, including the paper guides, nip rollers, photoreceptor drum, exit guides, and printer housing. The adhesive build-up inside the printer causes paper jams and poor print quality, and requires the printer to be serviced more frequently. Adhesive build-up in printers is especially a problem for high volume users of self-adhesive labels such as address labels.
Some attempts have been made to address the problem of edge tackiness and adhesive build-up in printers. One approach has been to coat only a portion of the release liner, leaving a gap where the sheet edges are to be cut, so that the final 8xc2xdxc3x9711  in. sheet has no adhesive coated within xe2x85x9c in. of the sheet edge. This approach requires more expensive processing of the label construction. Another approach has been to matrix strip the label sheets, i.e., remove adhesive and face stock at the edge of the release liner. This approach similarly requires additional processing. What is needed is a PSA specifically designed to leave very little adhesive residue in the printers without sacrificing adhesion to envelopes, file folders, computer diskettes, audio/video tapes, corrugated cardboard, polyethylene, and other substrates.
According to the present invention, there are provided PSAs and PSA constructions that exhibit good slittability and reduced adhesive build-up in printers, even after thousands of sheets of such constructions have been fed through a printer. Preferably, the PSAs have a dynamic storage modulus, Gxe2x80x2, at 1000 radians/s and 25xc2x0 C., of about 5xc3x97106 dyne/cm2 or higher, pre about 6xc3x97106 dyne/cm2 or higher. If the modulus is greater than 20xc3x97106 dyne/cm2, the adhesion properties begin to decrease. Additionally, the PSAs have a creep (% strain) at 90xc2x0 C. of about 125 or less, preferably about 100 or less, more preferably 60 or less, still more preferably 50 or less.
Storage modulus (Gxe2x80x2) of an adhesive is a measure of the elastic component of the modulus, or the energy stored and recovered in a dynamic mechanical measurement. Adhesive compositions with a sufficiently high storage modulus are less likely to be displaced by and less likely to adhere to cutting blades or dies used in slitting and converting processes. Excellent slittability has been achieved by confining the storage modulus of the PSA used in the present invention to about 5xc3x97106 dyne/cm2 or higher.
Creep (% strain) is a measure of the deformation over time of an adhesive under a given stress at a constant temperature. Adhesive compositions with a relatively low creep have been found to exhibit better printer performance, i.e., result in less adhesive build-up in a laser printer.
In one embodiment of the invention, a new PSA comprises the polymerization product of a plurality of monomers, comprising, on a percent by weight basis, based on the total weight of monomers, (a) about 25 to 35% of at least one alkyl acrylate; (b) about 30 to 50% of a diester of a dicarboxylic acid; (c) about 20 to 30% of a vinyl monomer; (d) about 0.5 to 5% of an ethylenically unsaturated C3 to C5 carboxylic acid; (e) about 0.15 to 1% of a cross-linking monomer selected from the group consisting of multifunctional acrylate and methacrylate, epoxy-functionalized acrylate and methacrylate, and chelating acrylate and methacrylate; and (f) about 1 to 5% of a carboxy acrylate or methacrylate monomer, the homopolymer of which has a glass transition temperature below that of poly(acrylic acid).
In one embodiment, a PSA construction is made by applying the polymerization product to a flexible release liner or face stock and, in the case of an emulsion or solvent polymer, driving off the water or solvent. Linerless PSA constructions can also be made with the PSAs described herein.
The invention is also directed to a method for minimizing adhesive build-up in a printer caused by extended printing of adhesive labels. The method comprises selecting a PSA determined to have a storage modulus of at least about 5xc3x97106 dyne/cm2 at 1000 radians/s and 25xc2x0 C. and a creep (% strain) at 90xc2x0 C. of less than about 125. A label construction comprising a release liner, the PSA coated on or applied to the release liner, and a face stock laminated to the PSA is prepared. The label construction is passed through a printer.
In another embodiment, the invention is directed to a method for minimizing adhesive build-up on a slitting blade during a PSA label-converting operation. The method comprises selecting a PSA determined to have a storage modulus of at least about 5xc3x97106 dyne/cm2 at 1000 radians/s and 25xc2x0 C. and a creep (% strain) at 90xc2x0 C. of less than about 125. A label construction comprising a release liner, the PSA coated on or applied to the release liner, and a face stock laminated to the PSA is prepared. The label construction is slit with the blade, whereby the blade is in contact with the release liner, the PSA and the face stock.